A) Field of the Invention
The present invention relates to a semiconductor device having capacitors and its manufacture method, and more particularly to a semiconductor device having capacitors disposed on conductive plugs formed in an interlayer insulating film and its manufacture method.
B) Description of the Related Art
A tendency of high-speed processing or storing a large amount of data has increased recently, with development of digital technologies. From this reason, semiconductor devices used in electronic apparatus are required to have high integration and high performance.
In order to realize high integration of a semiconductor memory device, e.g., a DRAM, instead of using conventional silicon oxide or silicon nitride, a variety of studies have been made on technologies of using ferroelectric material or high dielectric constant material as the material of a dielectric film of a capacitor constituting DRAM.
Vigorous studies have been made also on technologies of using a ferroelectric film having spontaneous polarization as a capacitor dielectric film, in order to realize a nonvolatile RAM capable of reading and writing data at low voltage and at high speed. The semiconductor storage device of this type is called a ferroelectric random access memory (FeRAM).
A ferroelectric memory stores data by utilizing hysteresis characteristics of ferroelectric substance. A ferroelectric memory has a ferroelectric capacitor which is constituted of a ferroelectric film and a pair of electrodes sandwiching the ferroelectric film. The ferroelectric film generates polarization corresponding to a voltage applied across the electrodes, and spontaneous polarization is left even the applied voltage is removed. As the polarity of an applied voltage is reversed, the polarity of spontaneous polarization is also reversed. Data can be stored by relating two polarities of spontaneous polarization to data “0” and “1”, respectively. Data can be read by detecting the polarity of spontaneous polarization. As compared to a flash memory, a ferroelectric memory can operate at a lower voltage and can write data at higher speed and with a smaller electric power.
A ferroelectric film constituting a capacitor of a ferroelectric memory is made of lead zirconate titanate (PZT), La-doped PZT (PLZT), PZT-based material finely doped with Ca, Sr or Si, Bi layer structure compound such as SrBi2Ta2O9 (SBT, Y1) and SrBi2(Ta, Nb)2O9 (SBTN, YZ) or the like. A ferroelectric film is formed by a sol-gel method, sputtering, metal organic chemical vapor deposition (MOCVD) or the like.
Generally, by using these film forming methods, a ferroelectric film in an amorphous phase or in a microcrystal phase is formed on a lower electrode. The crystalline structure is transformed into a perovskite structure or a bismuth layer structure by succeeding heat treatment. It is necessary to use, as the electrode material of a capacitor, material hard to be oxidized or material capable of retaining conductivity even if it is oxidized. Platinum group metal such as platinum (Pt) or iridium (Ir), and its oxide such as iridium oxide (IrOx) are generally and widely used as the electrode material. Aluminum (Al) is generally used as a wiring material just like general semiconductor devices.
A ferroelectric memory is required to have higher integration and higher performance just like other semiconductor devices, so a cell area is required to be reduced further in the near future. In order to reduce a cell area, it is effective to adopt a stack structure instead of a planar structure. The “stack structure” is a structure that a capacitor is disposed just above a conductive plug (contact plug) formed on the drain of a transistor constituting a memory cell. In a ferroelectric memory of a conventional stack structure, a capacitor has a structure that a barrier metal film, a lower electrode, a ferroelectric film and an upper electrode are laminated in this order just above a conductive plug made of tungsten (W) or the like. The barrier metal film has a role of preventing oxidation of the conductive plug. Since material is selected often which provides both the function of a barrier material film and the function of a lower electrode, it is difficult to definitely distinguish between the barrier metal film and lower electrode. Generally, the barrier metal film and lower electrode are made of a combination of two or more films selected from a group consisting of a TiN film, a TiAlN film, an Ir film, an IrO2 film, a Pt film and a SRO (SrRuO3) film.
It is important to control orientation of a ferroelectric film so as to be oriented uniformly to manufacture a ferroelectric memory having good electric characteristics and high product yield. Orientation of the ferroelectric film is influenced greatly by orientation of a lower electrode. Namely, by controlling orientation of the lower electrode so as to be oriented uniformly, orientation of the ferroelectric film can be made uniform. Therefore, in order to manufacture a ferroelectric memory having good electric characteristics and high product yield, it is important to control orientation of the lower electrode so as to be oriented uniformly.
JP-A-2000-91511 discloses a method of executing rapid thermal annealing of a lower electrode in inert gas in a temperature range between about 400° C. and 900° C. after the lower electrode is formed and before a ferroelectric film is deposited, in order to maximize spontaneous polarization of a ferroelectric capacitor.
JP-A-HEI 11-168174 discloses a method of executing heat treatment in a temperature range between 200° C. and 300° C. or at a higher temperature after a lower electrode made of Ir is formed, in order to make residual polarization characteristics of a ferroelectric film uniform over the whole substrate surface and to prevent cracks from being generated in the ferroelectric film.
JP-A-2002-151656 discloses a method of executing heat treatment at a temperature of 550° C. after a lower electrode having a two-layer structure of IrOx and Ir is formed and before a PZT film is formed, in order to prevent deterioration of the electric characteristics of the ferroelectric film made of PZT.
The methods disclosed in JP-A-2004-47633 will be described below. After a Ru film is formed which is used as a lower electrode of a ferroelectric capacitor, preliminary heat treatment is executed at 350° C. to 500° C. Thereafter, after a PZT film and the like are formed, heat treatment is executed at about 600° C. to 700° C. for crystallization. If the preliminary heat treatment is not executed, crystals of the Ru film grow abruptly during crystallization heat treatment and the Ru film has crystal grains irregular in size. If the preliminary heat treatment is executed, abrupt crystal growth can be suppressed during crystallization heat treatment and the crystal structure having grains equal in size can be maintained.